/* ----------------------------------------------------------------------
* Copyright (C) 2010 ARM Limited. All rights reserved.
*
* $Date:        15. February 2012
* $Revision: 	V1.1.0
*
* Project: 	    CMSIS DSP Library
* Title:	    arm_fir_decimate_fast_q31.c
*
* Description:	Fast Q31 FIR Decimator.
*
* Target Processor: Cortex-M4/Cortex-M3
*
* Version 1.1.0 2012/02/15
*    Updated with more optimizations, bug fixes and minor API changes.
*
* Version 1.0.10 2011/7/15
*    Big Endian support added and Merged M0 and M3/M4 Source code.
*
* Version 1.0.3 2010/11/29
*    Re-organized the CMSIS folders and updated documentation.
*
* Version 1.0.2 2010/11/11
*    Documentation updated.
*
* Version 1.0.1 2010/10/05
*    Production release and review comments incorporated.
*
* Version 1.0.0 2010/09/20
*    Production release and review comments incorporated.
* -------------------------------------------------------------------- */

#include "arm_math.h"

/**
 * @ingroup groupFilters
 */

/**
 * @addtogroup FIR_decimate
 * @{
 */

/**
 * @brief Processing function for the Q31 FIR decimator (fast variant) for Cortex-M3 and Cortex-M4.
 * @param[in] *S points to an instance of the Q31 FIR decimator structure.
 * @param[in] *pSrc points to the block of input data.
 * @param[out] *pDst points to the block of output data
 * @param[in] blockSize number of input samples to process per call.
 * @return none
 *
 * <b>Scaling and Overflow Behavior:</b>
 *
 * \par
 * This function is optimized for speed at the expense of fixed-point precision and overflow protection.
 * The result of each 1.31 x 1.31 multiplication is truncated to 2.30 format.
 * These intermediate results are added to a 2.30 accumulator.
 * Finally, the accumulator is saturated and converted to a 1.31 result.
 * The fast version has the same overflow behavior as the standard version and provides less precision since it discards the low 32 bits of each multiplication result.
 * In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits (where log2 is read as log to the base 2).
 *
 * \par
 * Refer to the function <code>arm_fir_decimate_q31()</code> for a slower implementation of this function which uses a 64-bit accumulator to provide higher precision.
 * Both the slow and the fast versions use the same instance structure.
 * Use the function <code>arm_fir_decimate_init_q31()</code> to initialize the filter structure.
 */

void arm_fir_decimate_fast_q31(
    arm_fir_decimate_instance_q31* S,
    q31_t* pSrc,
    q31_t* pDst,
    uint32_t blockSize)
{
	q31_t* pState = S->pState;                     /* State pointer */
	q31_t* pCoeffs = S->pCoeffs;                   /* Coefficient pointer */
	q31_t* pStateCurnt;                            /* Points to the current sample of the state */
	q31_t x0, c0;                                  /* Temporary variables to hold state and coefficient values */
	q31_t* px;                                     /* Temporary pointers for state buffer */
	q31_t* pb;                                     /* Temporary pointers for coefficient buffer */
	q31_t sum0;                                    /* Accumulator */
	uint32_t numTaps = S->numTaps;                 /* Number of taps */
	uint32_t i, tapCnt, blkCnt, outBlockSize = blockSize / S->M;  /* Loop counters */
	uint32_t blkCntN2;
	q31_t x1;
	q31_t acc0, acc1;
	q31_t* px0, *px1;

	/* S->pState buffer contains previous frame (numTaps - 1) samples */
	/* pStateCurnt points to the location where the new input data should be written */
	pStateCurnt = S->pState + (numTaps - 1u);

	/* Total number of output samples to be computed */

	blkCnt = outBlockSize / 2;
	blkCntN2 = outBlockSize - (2 * blkCnt);

	while(blkCnt > 0u) {
		/* Copy decimation factor number of new input samples into the state buffer */
		i = 2 * S->M;

		do {
			*pStateCurnt++ = *pSrc++;

		} while(--i);

		/* Set accumulator to zero */
		acc0 = 0;
		acc1 = 0;

		/* Initialize state pointer */
		px0 = pState;
		px1 = pState + S->M;

		/* Initialize coeff pointer */
		pb = pCoeffs;

		/* Loop unrolling.  Process 4 taps at a time. */
		tapCnt = numTaps >> 2;

		/* Loop over the number of taps.  Unroll by a factor of 4.
		 ** Repeat until we've computed numTaps-4 coefficients. */
		while(tapCnt > 0u) {
			/* Read the b[numTaps-1] coefficient */
			c0 = *(pb);

			/* Read x[n-numTaps-1] for sample 0 sample 1 */
			x0 = *(px0);
			x1 = *(px1);

			/* Perform the multiply-accumulate */
			acc0 = (q31_t)((((q63_t) acc0 << 32) + ((q63_t) x0 * c0)) >> 32);
			acc1 = (q31_t)((((q63_t) acc1 << 32) + ((q63_t) x1 * c0)) >> 32);

			/* Read the b[numTaps-2] coefficient */
			c0 = *(pb + 1u);

			/* Read x[n-numTaps-2]  for sample 0 sample 1  */
			x0 = *(px0 + 1u);
			x1 = *(px1 + 1u);

			/* Perform the multiply-accumulate */
			acc0 = (q31_t)((((q63_t) acc0 << 32) + ((q63_t) x0 * c0)) >> 32);
			acc1 = (q31_t)((((q63_t) acc1 << 32) + ((q63_t) x1 * c0)) >> 32);

			/* Read the b[numTaps-3] coefficient */
			c0 = *(pb + 2u);

			/* Read x[n-numTaps-3]  for sample 0 sample 1 */
			x0 = *(px0 + 2u);
			x1 = *(px1 + 2u);
			pb += 4u;

			/* Perform the multiply-accumulate */
			acc0 = (q31_t)((((q63_t) acc0 << 32) + ((q63_t) x0 * c0)) >> 32);
			acc1 = (q31_t)((((q63_t) acc1 << 32) + ((q63_t) x1 * c0)) >> 32);

			/* Read the b[numTaps-4] coefficient */
			c0 = *(pb - 1u);

			/* Read x[n-numTaps-4] for sample 0 sample 1 */
			x0 = *(px0 + 3u);
			x1 = *(px1 + 3u);


			/* Perform the multiply-accumulate */
			acc0 = (q31_t)((((q63_t) acc0 << 32) + ((q63_t) x0 * c0)) >> 32);
			acc1 = (q31_t)((((q63_t) acc1 << 32) + ((q63_t) x1 * c0)) >> 32);

			/* update state pointers */
			px0 += 4u;
			px1 += 4u;

			/* Decrement the loop counter */
			tapCnt--;
		}

		/* If the filter length is not a multiple of 4, compute the remaining filter taps */
		tapCnt = numTaps % 0x4u;

		while(tapCnt > 0u) {
			/* Read coefficients */
			c0 = *(pb++);

			/* Fetch 1 state variable */
			x0 = *(px0++);
			x1 = *(px1++);

			/* Perform the multiply-accumulate */
			acc0 = (q31_t)((((q63_t) acc0 << 32) + ((q63_t) x0 * c0)) >> 32);
			acc1 = (q31_t)((((q63_t) acc1 << 32) + ((q63_t) x1 * c0)) >> 32);

			/* Decrement the loop counter */
			tapCnt--;
		}

		/* Advance the state pointer by the decimation factor
		 * to process the next group of decimation factor number samples */
		pState = pState + S->M * 2;

		/* The result is in the accumulator, store in the destination buffer. */
		*pDst++ = (q31_t)(acc0 << 1);
		*pDst++ = (q31_t)(acc1 << 1);

		/* Decrement the loop counter */
		blkCnt--;
	}

	while(blkCntN2 > 0u) {
		/* Copy decimation factor number of new input samples into the state buffer */
		i = S->M;

		do {
			*pStateCurnt++ = *pSrc++;

		} while(--i);

		/* Set accumulator to zero */
		sum0 = 0;

		/* Initialize state pointer */
		px = pState;

		/* Initialize coeff pointer */
		pb = pCoeffs;

		/* Loop unrolling.  Process 4 taps at a time. */
		tapCnt = numTaps >> 2;

		/* Loop over the number of taps.  Unroll by a factor of 4.
		 ** Repeat until we've computed numTaps-4 coefficients. */
		while(tapCnt > 0u) {
			/* Read the b[numTaps-1] coefficient */
			c0 = *(pb++);

			/* Read x[n-numTaps-1] sample */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 = (q31_t)((((q63_t) sum0 << 32) + ((q63_t) x0 * c0)) >> 32);

			/* Read the b[numTaps-2] coefficient */
			c0 = *(pb++);

			/* Read x[n-numTaps-2] sample */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 = (q31_t)((((q63_t) sum0 << 32) + ((q63_t) x0 * c0)) >> 32);

			/* Read the b[numTaps-3] coefficient */
			c0 = *(pb++);

			/* Read x[n-numTaps-3] sample */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 = (q31_t)((((q63_t) sum0 << 32) + ((q63_t) x0 * c0)) >> 32);

			/* Read the b[numTaps-4] coefficient */
			c0 = *(pb++);

			/* Read x[n-numTaps-4] sample */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 = (q31_t)((((q63_t) sum0 << 32) + ((q63_t) x0 * c0)) >> 32);

			/* Decrement the loop counter */
			tapCnt--;
		}

		/* If the filter length is not a multiple of 4, compute the remaining filter taps */
		tapCnt = numTaps % 0x4u;

		while(tapCnt > 0u) {
			/* Read coefficients */
			c0 = *(pb++);

			/* Fetch 1 state variable */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 = (q31_t)((((q63_t) sum0 << 32) + ((q63_t) x0 * c0)) >> 32);

			/* Decrement the loop counter */
			tapCnt--;
		}

		/* Advance the state pointer by the decimation factor
		 * to process the next group of decimation factor number samples */
		pState = pState + S->M;

		/* The result is in the accumulator, store in the destination buffer. */
		*pDst++ = (q31_t)(sum0 << 1);

		/* Decrement the loop counter */
		blkCntN2--;
	}

	/* Processing is complete.
	 ** Now copy the last numTaps - 1 samples to the satrt of the state buffer.
	 ** This prepares the state buffer for the next function call. */

	/* Points to the start of the state buffer */
	pStateCurnt = S->pState;

	i = (numTaps - 1u) >> 2u;

	/* copy data */
	while(i > 0u) {
		*pStateCurnt++ = *pState++;
		*pStateCurnt++ = *pState++;
		*pStateCurnt++ = *pState++;
		*pStateCurnt++ = *pState++;

		/* Decrement the loop counter */
		i--;
	}

	i = (numTaps - 1u) % 0x04u;

	/* copy data */
	while(i > 0u) {
		*pStateCurnt++ = *pState++;

		/* Decrement the loop counter */
		i--;
	}
}

/**
 * @} end of FIR_decimate group
 */
